Almost as old as the Fischer-Tropsch process for making hydrocarbons is the Fischer-Tropsch process for making alcohols. The reaction is carried out by passing a mixture of carbon monoxide and hydrogen over a catalyst for the hydrogenation of the carbon monoxide. A typical review article is R. B. Anderson et al., Industrial and Engineering Chemistry, Vol. 44, No. 10, pp. 2418-2424. This paper lists a number of catalysts containing zinc, copper, chromium, manganese, thorium, iron, occasionally promoted with alkali or other materials for making various alcohols. The authors state that ethyl alcohol is a major constituent, the yield of methanol is usually very small and a tentative summary of factors favouring the production of alcohols is high pressure, low temperature, high space velocity, high recycle ratio and carbon monoxide-rich synthesis gas.
Molybdenum is known to be catalytic for the Fischer-Tropsch process and is taught in U.S. Pat. No. 4,151,190 and U.S. Pat. No. 4,199,522 which are incorporated herein by reference. The references describe some of the herein used catalysts but do not teach that the catalyst is useful for making commercially significant quantities of alcohols. These references note that hydrogen sulfide affects the activity of the catalyst.
British patent publication No. 2,065,491 discloses a process for making C.sub.2 hydrocarbons from H.sub.2 /CO using a catalyst comprising a group VB and/or VIB element in combination with an iron group metal as free metals, oxides or sulfides on a porous oxidic support. The authors note that the presence of H.sub.2 S alters the activity and selectivity of their process.
U.S. Pat. No. 4,177,202 discloses a process for making hydrocarbons from H.sub.2 /CO over a molybdena and optionally cobalt or vanadium catalyst. Selectivity to ethane is enhanced by presence of hydrogen sulfide in the feed.
U.S. Pat. No. 2,490,488 discloses that molybdenum sulfide methanation catalysts acquire Fischer-Tropsch activity when promoted with an alkaline compound of an alkali metal. The example of the invention shows a 30 percent selectivity to C.sub.3 +hydrocarbons and oxygenates. Of this 30 percent, no more than 44 percent boils near or above 65.degree. C., the boiling point of methanol. Accordingly, the maximum possible alcohol selectivity is no more than 13.2 percent (44 percent of 30 percent).
U.S. Pat. No. 2,539,414 describes a Fischer-Tropsch process with molybdenum carbide catalysts. It teaches that the catalyst may be used to form oxygenates and at column 3, lines 66-71 teaches that one might get alcohols or hydrocarbons by varying the conditions.
G. T. Morgan et al., J. Soc. Chem. Ind., Vol. 51, Jan. 8, 1932, pp. 1T-7T describe a process for making alcohols with chromium/manganese oxide catalysts promoted with alkali.
A number of references teach production of alcohols using rhodium catalysts. Some of these contain molybdenum as an optional ingredient. U.S. Pat. No. 4,014,913 discloses a catalyst containing rhodium and thorium or uranium and iron or molybdenum or tungsten for the production of ethanol. U.S. Pat. No. 4,096,164 discloses the use of rhodium in combination with molybdenum or tungsten and Example A discloses that use of a molybdenum-on-silica catalyst yielded 4.4 percent oxygenates.
EPO application No.81-33,212 (Chemical Abstracts 96:51, 800a) discloses a similar process using rhodium in combination with one or more of a long list of metals which includes molybdenum.
All of the aforementioned references are hereby incorporated by reference.
U.S. application Ser. No. 474,674, filed Mar. 18, 1984, which is hereby incorporated by reference, discloses a process for producing alcohols from synthesis gas using molybdenum and tungsten catalysts. In this application it is noted that these catalysts are tolerant to hydrogen sulfide in the hydrogen/carbon monoxide feed.
U.S. application Ser. No. 622,029, filed June 18, 1984, which is hereby incorporated by reference is a continuation-in-part of U.S. Ser. No. 476,674. In it are disclosed the catalysts in the process of this invention.
To make a commercially significant alcohol process, one must use a catalyst and conditions which are highly efficient. To be efficient the catalyst must yield a high weight ratio of product per unit weight of catalyst in a given period of time. The catalyst must be stable and active for long periods of time between regenerations. This may be particularly difficult to accomplish when the H.sub.2ratio of the feed gas is low, such as less than 2 to 1. Ideally the catalyst will be sulfur tolerant and will have a high selectivity to a commercial product to avoid purification or removal and disposal of by-products and to avoid separation into two or more product streams.
When the mixed alcohols product is to be used as a fuel replacement or a fuel additive it may be desirable that the ratio of C.sub.1 to C.sub.2 + alcohols be no greater than a certain amount. As used in this Application the ratio of C.sub.1 to C.sub.2 + alcohols means the molar ratio of methanol to C.sub.2 to C.sub.5 alcohols such as ethanol, propanols, butanols and pentanols, taken as a whole. The term, alcohols, generally does not include alcohols present as esters or aldehydes. For example, the methanol portion of methyl acetate is not counted as methanol.
Excessive methanol is generally considered an unattractive additive to gasolines. Methanol may decrease drivability and may increase corrosion in the fuel system and may cause phase separation when used in excessive quantities. These problems may be alleviated by blending methanol with higher alcohols.
Accordingly one may wish to synthesize mixed alcohols with no more than a certain amount of methanol in the blend. Or in a similar fashion one may wish to minimize the ratio of C.sub.1 to C.sub.2 + alcohols in mixed alcohols so that methanol may be purchased and blended into the mixed alcohols to give the maximum acceptable C.sub.1 to C.sub.2 + alcohols ratio.
Up until now, a method of varying the ratio of C.sub.1 to C.sub.2 + alcohols short of changing catalysts or distilling the product stream has not been known. Since distillation adds an additional process step, it is desirable to be able to vary the C.sub.1 to C.sub.2 + alcohols ratio that is produced in the reaction to form the alcohols.